February 20, 2013
Scientists Use Radio Telescopes And GPS To Monitor Nuclear Tests
Alan McStravick for redOrbit.com — Your Universe Online
The world was on edge earlier this month when North Korea detonated an underground nuclear explosion (UNE) meant to showcase to the world community the abilities of this beleaguered nation. In a recent story published in The Guardian, reports surfaced North Korea is planning two additional nuclear tests just this year. UNEs were once commonplace, giving the testing nation the knowledge surrounding their nuclear capability. The US performed their last UNE in September of 1992 in the Nevada desert.
This last UNE performed by the US, codenamed Hunters Trophy, was analyzed by a team comprised of astronomers and researchers at the US Naval Research Laboratory and The Ohio State University Department of Civil, Environmental and Geodetic Engineering. The team utilized both radio telescope interferometry and Global Positioning Satellite (GPS) data recorded of the ionosphere.
Popularized in the movie ℠Contact´, the Very Large Array (VLA), operated by the National Radio Astronomy Observatory, with their 27 individual 25-meter parabolic dish antennas, was used to collect the radio telescope data for the study. The VLA is located 50 miles to the west of Socorro, New Mexico, situated in the Plains of San Agustin.
The VLA, as an interferometer, operates by multiplying the data from each pair of telescopes. This multiplication allows for the observation of interference patterns. It is the structure of interference patterns, and the changes they undergo in time due to the Earth´s rotation, that reflects the structure of radio sources in the sky.
The research team utilized the VLA to act in a manner that is contrary to its typical application. As a radio synthesis telescope meant to observe bands between 1 and 50 gigahertz, the VLA is most usually used in the observation of cosmic sources. A strict detailed calibration scheme is required to remove the effects of the ionosphere when searching the cosmos. However, for this study, the researchers wanted to direct their focus completely upon the ionosphere.
On September 18, 1992, during the Hunters Trophy UNE, the VLA was in the process of observing several relatively bright cosmic sources at 1.4 gigahertz. The arrangement of the array in a compact D-configuration allowed for an increased sensitivity to smaller-scale fluctuations. This allowed the team to mine the archived data for that day. A self-calibration was performed by using each source to obtain differences in total electron content (ÎTEC) time series for each antenna.
"One can see that between roughly 20 and 25 minutes after the UNE, the signature of disturbances in the ionosphere moving toward the northeast and southwest are nearly perpendicular to the direction from Hunters Trophy," said US Naval Research Laboratory radio astronomer Joseph Helmboldt, PhD. "Given their characteristics, it seems likely that these disturbances are associated with small-scale distortions propagating along the larger traveling ionospheric disturbance generated by the Hunters Trophy explosion."
The use of GPS is also very beneficial in the study of the detection and discrimination of UNEs. This is due, mainly, to the sensitivity of GPS signals to ionospheric disturbances. GPS measurements were used for the validation of the VLA measurements. The team believed, by studying the ionosphere, and more specifically Traveling Ionospheric Disturbances (TIDs) that were excited by acoustic-gravity waves from surface and underground explosions, they would be able to find an increase in fluctuation some 30 minutes after the detonation. With GPS data supporting their findings, the team contends both travel time and distance of each event were highly correlated.
"Combining the results of both spectral techniques, we found a class of intermediate and small-scale waves," said Helmboldt. "Our exploration of VHF/UHF observations with the VLA has successfully demonstrated the power of this instrument to characterize a variety of transient ionospheric phenomena."
The US Naval Research Laboratory, established over 85 years ago and located in the Washington, DC area, is a full-spectrum corporate laboratory engaged in a broadly based multidisciplinary program of scientific research and advanced technological development. Other major sites for the laboratory are located at the Stennis Space Center in Mississippi and in Monterey, Califoria.